The present invention relates to improved dies for the extrusion of honeycomb structures from plasticized organic or inorganic batch materials. More particularly, the invention relates to a honeycomb extrusion die incorporating a batch stressor which provides improved batch homogeneity, and methods for making and using the die. A batch stressor as defined herein, is a section of a significantly reduced flow channel located virtually immediately upstream of the slot matrix of a die.
The use of extrusion dies to form thin-walled honeycomb structures is well known in the art. U.S. Pat. Nos. 3,790,654 and 3,905,743 to Bagley describe one design for such a die, that design incorporating a plurality of feedholes entering an inlet face of the die and extending through the body of the die to convey extrudable material to a discharge section formed in the die outlet surface by an array of discharge slots. The discharge slots, formed between a plurality of pins interconnect with each other, reforming the extrudable material into an interconnected wall structure for a channeled honeycomb body as the material is discharged from the outlet face of the die.
As the uses for such honeycomb structures have increased, so also has the need for extrusion dies capable of forming more finely structured honeycombs. Die development has also focused on ways of obtaining a more uniform flow and proper distribution of extrudable material to the outlet face of the die. For example, published Japanese Utility Model applications 52-8761 and 52-8762 disclose discrete channels for the distribution of feed streams directly to slot locations on the outlet face, while U.S. Pat. No. 4,242,075 describes a die construction with feed material distribution channels feeding an array of spoke-supported cell blocks for forming the cells in the extruded honeycomb.
The problem of smoothly conveying a batch material from a feedhole inlet to a discharge slot outlet can be better appreciated when it is recognized that batch flow is longitudinal through the feedhole section of the die, but rapidly transitions to a combination of lateral and longitudinal flow at the feedhole/discharge slot interface. Rapid lateral flow at these junctions is required to adequately fill the discharge slot array, but if the flow is non-uniform, defects such as marginal cell wall knitting, wavy or swollen cell walls, missing cell walls, and plugged cell walls will appear in the extruded product.
Feedhole characteristics, such as diameter, surface finish, spiraling, etc., differ from one hole to another within a given die, and thus have a noticeable affect on the quality of the extruded substrate. Thin wall products tend to be especially sensitive to these non-uniform feedhole conditions. In addition, super smooth slot finishes generated by diamond wheel grinding are routinely available today, and the improved pin surfaces forming the slots further sensitize the die to the influences of non-uniform feedholes.
Non-uniform flow into the discharge slots and along the pin surface of the die is responsible for web defects in the extruded substrate. Gross defects show up as thick or swollen webs, while more subtle forms emerge as shadows. In electrical chemical machining (ECM) drilled body plates, these imperfections appear as banding along entire rows of feedholes, reflecting the positioning and advancing of the ECM tube holders as they are indexed across the blank during the drilling operations. In gun-drilled body plates, the flaws are organized into patchy areas which appear to track subtle differences in tooling quality and tooling changes. Extrusion dies for producing thin web honeycomb products are especially sensitive to flow conditions which exacerbate the degree and frequency of the defects caused by non-uniform batch flow.
The art has long recognized the desirability of smoothing the feed channels in an extrusion die to reduce back pressure and to reduce abrasive wear on the die caused by the inorganic powder mixtures being extruded. U.S. Pat. No. 5,066,215 describes a die wherein the feedholes uniformly taper to discharge slots on the die outlet surface. U.S. Pat. No. 3,846,197 also discloses a similar gradual transitioning of feedholes and discharge slots. In U.S. Pat. No. 5,702,659 the problem of material flow control at the feedhole-slot interface of a honeycomb extrusion die is addressed by fashioning the interface with a stack of thin plates. Each plate in the stack contains multiple openings representing a very thin cross-section of the desired flow channel within the die at that point. Such approach permits each successive plate in the stack to have its own unique geometry, and the stack transitions the flow of batch material from the feedholes to the discharge slots.
In the light of these continuing difficulties, it is a principal object of the present invention to provide an extrusion die that more effectively addresses many of the problems presented by conventionally constructed extrusion dies.
It is a further object of the invention to provide an extrusion method utilizing an improved die which uniformizes the batch material entering the discharge slots to provide extruded honeycombs of improved quality.
Other objects and advantages of the invention will become apparent from the following description thereof.
In accordance with the present invention, the problem of non-uniform material flow to the discharge slots of a honeycomb extrusion die is addressed by fashioning a batch stressor between the feedholes and the discharge slots of the die. The batch stressor causes a discontinuity in normal flow by directing batch through a venturi-like section adjacent the end of each feedhole. The effect of the batch stressor is to suddenly increase linear flow velocity due to the reduced cross-sectional area, and then just as suddenly to restore the original conditions of flow. The rapid change in particle alignment due to the cross-sectional change produces a shearing influence on the batch and essentially uniformizes or makes the batch more homogeneous as it enters the transition area or discharge slot sections of the die, significantly reducing the frequency of shadows and other web defects cause by non-uniform batch flow.
When a relatively thin component having smaller diameter holes than the diameter of the feedholes is placed across the feedhole flow, it results in added impedance due primarily to the hole diameter rather than the hole length or surface finish. Since it is easier to control hole diameter than it is surface finish during the machining operations, a means becomes available to add uniform impedance that thus shears the batch to a uniform condition before it exits such component. However, if a batch stressor component were positioned adjacent the inlet end of a feedhole, the uniformity would gradually deteriorate as flow continues down the feedhole. That is, the batch gradually spreads out into intimate contact with the sidewalls of the die feedhole where the batch again comes under the influence of surface finish irregularities along the sidewalls. Thus, it can be seen that batch flow non-uniformity at the pin face of the die can be related to the differing characteristics of the feedholes. The present invention recognizes this problem and provides a solution thereto by installing a batch stressor, which performs similarly to an impedance plate, in the batch flow channel immediately at the outlet end of the feedholes and behind the face plate, to thereby deliver uniform batch into the relatively short transition section when utilized, or directly into the discharge slots adjacent the pin face of the die. Such configuration avoids contact with the relatively long path of flow of the feedhole, and its non-uniform impact on the flow stream.